Multipurpose tyre for a motor vehicle

ABSTRACT

A multipurpose tyre for a motor vehicle is provided with a tread ( 2; 52; 62; 162; 262; 362 ) which has a central region ( 3 ) comprising at least first and second blocks ( 22, 23; 66, 67 ) arranged between an equatorial plane ( 100 ) and a circumferential groove ( 14 ); the first and second blocks ( 22, 23; 66, 67 ) form a first and a second row ( 11, 12 ) and are separated from each other by first and second grooves ( 30, 31; 73, 74 ) which extend in an oblique direction relative to the equatorial plane ( 100 ); the first oblique grooves ( 30; 73 ) extend substantially perpendicularly relative to the second oblique grooves ( 31; 74 ); and each first oblique groove ( 30; 73 ) crosses two successive second oblique grooves ( 31; 74 ) and forms with each of them a respective junction which is essentially T-shaped ( 32, 33; 94, 95 ).

[0001] The present invention relates to a multipurpose tyre (all-seasonor all-road tyre) for a motor vehicle.

[0002] More specifically, the invention relates to the grooving of amultipurpose tyre, in as much as it is particularly suitable for motorcar tyres designed to give good performance both on dry roads and on wetroads, and even on snow-covered roads.

[0003] As is known, in tyres of multipurpose type the pattern and thecompound of the tread band are designed so as to satisfy the variousdemands specifically required with reference to the type of behaviourwhich the tyre needs to display on various types of surface, i.e. notonly on dry or wet surfaces but also on snow-covered surfaces, for whichits use is envisaged.

[0004] These requirements are mainly conflicting, as a result of whichthe tyres which have been produced to date normally represent acompromise between these requirements, but do not in any case, however,reach the performance levels obtained with a tyre specifically designedfor use on a given type of road surface, whether this is dry, wet orsnow-covered.

[0005] Generally speaking, irrespective of the use for which it isdesigned, a good tyre should have, inter alia, good properties ofdirectionality and tractional power, as well as good wear resistance.

[0006] The directionality properties, which consist of the ability ofthe tyre to accurately maintain the set trajectory, are positivelyinfluenced by the presence of longitudinal grooves, while the tractionalpower properties, which consist of the capacity of the tyre to transmittangential forces both during acceleration and when braking, areenhanced by providing grooves aligned transversely to the direction ofmotion. The efficacy of these transverse grooves as regards thetractional power is proportionately better the more the alignment of thegrooves approaches a direction parallel to the axis of rotation of thetyre, so that, normally, angles of between 45° and 90° relative to anequatorial plane of the tyre are chosen for these grooves.

[0007] That being said, it should be pointed out that as regards tyresspecifically designed for driving on dry surfaces, the number and widthof the longitudinal and transverse grooves tend to be reduced to improvethe wear resistance, the noise level in motion and the smooth rideprovided by the tyre.

[0008] However, as regards driving on a wet surface, a larger groovewidth is desired, especially as regards the circumferential grooves,which are responsible for discharging the water collected under theimprint area of the tyre in order to avoid the occurrence of thewell-known and hazardous aquaplaning effect.

[0009] Finally, as regards driving on snow-covered surfaces, a largenumber of small notches or grooves is required in order to adequatelytrap the snow picked up off the road surface, the snow-on-snow frictionbeing greater than the rubber-on-snow friction.

[0010] From the foregoing, it is quite easy to appreciate thedifficulties encountered in producing a multipurpose tyre whichsimultaneously has to satisfy all these conflicting requirements,arising specifically with regard to driving on dry, wet and snow-coveredsurfaces.

[0011] Specifically, the use of wide circumferential grooves conflictsboth with the requirements of driving on dry surfaces, which prefersmall grooves in order to increase the solid portion of the treadpattern, so as to improve the wear resistance and the noise level inmotion, and with the requirements of driving on snow-covered surfaces,with regard to which wide grooves, which aid the self-cleaning of thetread pattern, produce a limitation of the behavioral properties,particularly tractional power and road holding.

[0012] The many notches required for use on snow-covered surfaces alsoconflict with the requirements for driving on dry surfaces, in which anexcessive number of notches entails greater deformability of the blocks,with an increase in the noise level, and with the wear, as well as areduction in the driving stability and smoothness of ride.

[0013] Thus it follows that the current multipurpose tyres have quitegood hold under wet conditions, although this is inferior to that ofmodern tyres specifically designed for driving on wet surfaces, quitegood tractional power in snowy conditions, but inferior to that whichcan be achieved by tyres specifically designed for winter conditions, aswell as acceptable wear resistance and a sufficient hold on dry roads,although not to the same level as the best summer tyres commerciallyavailable.

[0014] The aim of the present invention is to produce a tyre of road andoff-road type which has excellent behaviour on snow-covered roads, aswell as with respect to aquaplaning and good behaviour on dry roads.

[0015] A first object of the invention is a multipurpose tyre for amotor vehicle, said tyre having an equatorial plane and being providedwith a tread which has a central region and a first and a secondshoulder region, said central region being separated from said first andsecond shoulder regions by a first and a second circumferential groove,respectively, said central region comprising at least first and secondblocks arranged between said equatorial plane and one of said first andsecond circumferential grooves, characterized in that said first andsecond blocks form a first and a second row and are separated from eachother by first and second grooves which extend in an oblique directionrelative to said equatorial plane, said first oblique grooves extendingsubstantially perpendicularly relative to said second oblique grooves,each first oblique groove crossing two successive second oblique groovesand forming with each of them a respective junction which is essentiallyT-shaped.

[0016] The tyre according to the invention has optimized tractionalpower and braking on snow-covered roads since the central region of itstread has a plurality of crossed oblique grooves with T-shapedjunctions, i.e. dead-end junctions, which trap the snow and enablesnow-on-snow friction to be produced.

[0017] Preferably, said first oblique grooves have a width whichdecreases in the direction of said equatorial plane. In fact, it hasbeen noted that this characteristic helps to trap the snow too.

[0018] Advantageously, said first and second blocks of said first andsecond rows are side by side and diverge obliquely.

[0019] Preferably, said first blocks have two portions which extendobliquely relative to each other, such that each block has an overallshape resembling that of the number 1.

[0020] In turn, said second blocks are essentially trapezoid-shaped.

[0021] Preferably, said central region also comprises third and fourthblocks located between said equatorial plane and the other of said firstand second circumferential grooves, said third and fourth blocks forminga third and a fourth row and being separated from each other by thirdand fourth grooves which extend in an oblique direction relative to saidequatorial plane, said third oblique grooves extending substantiallyperpendicularly relative to said fourth oblique grooves, each thirdoblique groove crossing two successive fourth oblique grooves andforming with each of them a respective junction which is essentiallyT-shaped.

[0022] Advantageously, said third oblique grooves have a width whichdecreases in the direction of said equatorial plane.

[0023] Preferably, said third and fourth blocks of said third and fourthrows are side by side and diverge obliquely, said third and fourthside-by-side blocks being inclined in the opposite direction to saidfirst and second side-by-side blocks.

[0024] Advantageously, said third blocks have two portions which extendobliquely relative to each other, such that each block has an overallshape resembling that of the number 1.

[0025] In turn, said fourth blocks are essentially trapezoid-shaped.

[0026] Preferably, at least two of said first blocks are also separatedfrom each other by a fifth oblique groove, which is substantiallyparallel to said second oblique grooves, said fifth oblique groovejoining up with one of said first oblique grooves at that end of thelatter which is nearest said equatorial plane, by means of an elbowjoin.

[0027] Furthermore, at least two of said first blocks are also separatedfrom each other by a first oblique sipe which is substantially parallelto said second oblique grooves, said first oblique sipe joining up withone of said first oblique grooves at that end of the latter which isnearest said equatorial plane.

[0028] Advantageously, at least two of said third blocks are alsoseparated from each other by a sixth oblique groove which issubstantially parallel to said fourth oblique grooves, said sixthoblique groove joining up with one of said third oblique grooves at thatend of the latter which is nearest said equatorial plane, by means of anelbow join.

[0029] Furthermore, at least two of said third blocks are also separatedfrom each other by a second oblique sipe, which is substantiallyparallel to said fourth oblique grooves, said second oblique sipejoining up with one of said third oblique grooves at that end of thelatter which is nearest said equatorial plane.

[0030] Preferably, said central region also comprises a circumferentialrib and a third and a fourth circumferential groove.

[0031] Advantageously, said third circumferential groove separates saidcircumferential rib from said row of first blocks and is incommunication with said first circumferential groove via firsttransverse zig-zag paths comprising said fifth oblique grooves and saidfirst and second oblique grooves.

[0032] In turn, said fourth circumferential groove separates saidcircumferential rib from said row of third blocks and is incommunication with said second circumferential groove via secondtransverse zig-zag paths comprising said sixth oblique grooves and saidthird and fourth oblique grooves.

[0033] Advantageously, said first circumferential groove is wider thansaid second circumferential groove.

[0034] In turn, said second oblique grooves are wider than said fourthoblique grooves.

[0035] Moreover, said third and fourth circumferential grooves are widerthan said first circumferential groove.

[0036] According to one embodiment, said first and second blocks haveareas that are substantially equal to the areas of said third and saidfourth blocks, respectively.

[0037] According to a further embodiment, said first and second blockshave areas that are substantially less than the areas of said third andsaid fourth blocks, respectively.

[0038] According to one embodiment, said circumferential rib has amid-plane which is coincident with said equatorial plane.

[0039] According to a further embodiment, said circumferential rib has amid-plane which is eccentric with respect to said equatorial plane.

[0040] Preferably, each of said first blocks has a pair of substantiallyoblique third sipes which are substantially parallel to said secondoblique grooves, and a fourth sipe which is essentially hook-shaped withsides that are substantially parallel to said first and said secondoblique grooves.

[0041] In turn, each of said second blocks has three substantiallyoblique fifth sipes which are substantially parallel to said secondoblique grooves.

[0042] In addition, each of said third blocks is free of sipes.

[0043] Advantageously, each of said fourth blocks has a sixth sipe whichis substantially trapezoid-shaped, similar to the shape of therespective fourth block which is substantially trapezoid-shaped andsmaller in size than this block.

[0044] According to one embodiment, each of said third blocks has threesubstantially oblique seventh sipes which are substantially parallel tosaid fourth oblique grooves.

[0045] In turn, each of said fourth blocks has a pair of substantiallyoblique eighth sipes which are substantially parallel to said fourthoblique grooves.

[0046] Advantageously, each of said first blocks has a ninth obliquesipe which is substantially parallel to said second oblique grooves, anda tenth sipe which is substantially S-shaped.

[0047] In turn, each of said second blocks has an eleventh central sipewhich is substantially S-shaped, and two substantially oblique twelfthsipes which are substantially parallel to said second oblique groovesand located on either side of said S-shaped eleventh sipe.

[0048] Preferably, each of said third blocks has two substantiallyoblique thirteenth sipes which are substantially parallel to said fourthoblique grooves, and a fourteenth sipe which is substantially S-shaped.

[0049] Furthermore, each of said fourth blocks has a central fifteenthsipe which is substantially S-shaped, and two substantially obliquesixteenth sipes which are substantially parallel to said fourth obliquegrooves and located on either side of said S-shaped fifteenth sipe.

[0050] Advantageously, said circumferential rib has seventeenth andeighteenth substantially oblique sipes which are, substantially parallelto said second and fourth oblique grooves.

[0051] Preferably, said first shoulder region comprises fifth shoulderblocks separated by seventh grooves which are substantially transverserelative to said equatorial plane and which communicate with said firstcircumferential groove, and said second shoulder region comprises sixthshoulder blocks separated by eighth grooves that are substantiallytransverse relative to said equatorial plane and which communicate withsaid second circumferential groove.

[0052] Advantageously, each seventh substantially transverse groove hasa connecting portion provided with an opening which is substantiallyeccentric relative to each second oblique groove.

[0053] In turn, each substantially transverse eighth groove has aconnecting portion provided with an opening which is substantiallyconcentric with a fourth oblique groove.

[0054] According to a further embodiment, each substantially transverseseventh groove is wider than each substantially transverse eighthgroove.

[0055] Preferably, each of said fifth shoulder blocks has threenineteenth sipes which are substantially transverse relative to saidequatorial plane.

[0056] In turn, each of said sixth shoulder blocks has a twentieth sipewhich is substantially transverse relative to said equatorial plane.

[0057] According to an embodiment, each of said sixth shoulder blockshas three twenty-first sipes which are substantially transverse relativeto said equatorial plane.

[0058] According to a further embodiment, each of said fifth shoulderblocks has two twenty-second substantially transverse sipes, each ofwhich crosses a substantially circumferential twenty-third sipe.

[0059] In turn, each of said sixth shoulder blocks has a centraltwenty-fourth sipe which is substantially S-shaped and two substantiallytransverse twenty-fifth sipes located on either side of said S-shapedtwenty-fourth sipe.

[0060] A second object of the invention is a multipurpose tyre for amotor vehicle, said tyre having an equatorial plane and being providedwith a tread having a central region and a first and a second shoulderregion, said central region being separated from said first and secondshoulder regions by a first and a second circumferential groove,respectively, said central region comprising at least first and secondblocks located between said equatorial plane and one of said first andsecond circumferential grooves, characterized in that said first andsecond blocks form a first and a second row and are separated from eachother by first and second grooves which extend in an oblique directionrelative to said equatorial plane, said first oblique grooves extendingessentially perpendicularly relative to said second oblique grooves,said first oblique grooves having a width which decreases in thedirection of said equatorial plane.

[0061] Preferably, said central region also comprises third and fourthblocks located between said equatorial plane and the other of said firstand second circumferential grooves, said third and fourth blocks forminga third and a fourth row and being separated from each other by thirdand fourth grooves which extend in an oblique direction relative to saidequatorial plane, said third oblique grooves extending essentiallyperpendicularly relative to said fourth oblique grooves, said thirdoblique grooves having a width which decreases in the direction of saidequatorial plane.

[0062] In the tyre according to the invention, the blocks in the centralregion and in the shoulder regions have a plurality of sipes whichcontribute towards trapping snow.

[0063] The tyre also has optimized behaviour with respect toaquaplaning. Contribution towards this is made by the presence, in theinner zone of the tread, of circumferential and transverse shouldergrooves which are wider than the analogous circumferential andtransverse grooves in the outer zone of the tread.

[0064] Furthermore, the tyre has good behaviour in terms of tractionalpower, braking and cornering on dry roads.

[0065] In the course of the present description and in the claims, theterm “rib” means a bead of a tyre tread band which is continuouslydelimited on two of its sides, and the term “sipe” means a notch notwider than 1 mm.

[0066] Further characteristics and advantages of the invention will nowbe illustrated with reference to embodiments shown by way ofnon-limiting example in the attached figures, in which

[0067]FIG. 1 is a perspective view of a multipurpose tyre made accordingto the invention;

[0068]FIG. 2 is a partial plan view of a tread of the tyre of FIG. 1;

[0069]FIG. 3 is a view in cross section on the plane marked III-III inFIG. 2;

[0070]FIG. 4 is a view in perspective of an embodiment of the tyre ofFIG. 1;

[0071]FIG. 5 is a partial plan view of a tread of the tyre of FIG. 4;

[0072]FIG. 6 is a view in cross section on the plane marked VI-VI inFIG. 5;

[0073]FIG. 7 is a partial plan view of a tread of a further embodimentof the tyre of FIG. 1;

[0074]FIG. 8 is a view in cross section on the plane marked VIII-VIII inFIG. 7;

[0075]FIGS. 9, 10 and 11 are partial plan views of embodiments of thetread of the tyre of FIGS. 7 and 8;

[0076] FIGS. 12-16 show treads for conventional tyres (X, Y, Z, V, W).

[0077]FIGS. 1, 2 and 3 show a multipurpose tyre 1 for a motor vehicle,with a tread 2 of a selected compound. Tread 2 is of the asymmetrictype, i.e. it works more efficiently when the tyre is mounted on thevehicle with a given orientation rather than with the oppositeorientation. In other words, the tyre preferably has an inner side wall(vehicle side) and an outer side wall. Tyre 1 has an equatorial plane100.

[0078] Tread 2 has a central region 3 and two shoulder regions 4 and 5,left and right. Central region 3 comprises a circumferential rib 6, twodeep circumferential grooves 7 and 8, and circumferential rows of blocks9, 10, 11 and 12. Rib 6 has a longitudinal mid-plane which is coincidentwith equatorial plane 100. Central region 3 is divided from shoulderregion 4 by a deep circumferential side groove 13 and from shoulderregion 5 by a deep circumferential side groove 14. Shoulder region 4comprises a circumferential row of shoulder blocks 15 and shoulderregion 5 comprises a circumferential row of shoulder blocks 16.

[0079] The circumferential rows of blocks 9 and 10 are located to theleft of equatorial plane 100 and the circumferential rows of blocks 11and 12 are located to the right of the equatorial plane. The row ofblocks 9 comprises blocks 20 which are substantially trapezoid-shaped(scalene trapezium) and the row of blocks 10 comprises blocks 21 havingtwo portions which extend obliquely relative to each other such thateach block has an overall shape resembling that of the number 1. Inturn, the row of blocks 11 comprises blocks 22 having two portions whichextend obliquely relative to each other such that each block has anoverall shape resembling that of the number 1 and the row of blocks 12comprises blocks 23 which are substantially trapezoid-shaped (scalenetrapezium). Blocks 20 and 21 of rows 9 and 10 which are side by side andblocks 22 and 23 of rows 11 and 12 which are side by side, divergeobliquely with an arrangement of the herringbone type. Side-by-sideblocks 20 and 21 of rows 9 and 10 have opposite inclinations to those ofside-by-side blocks 22 and 23 of rows 11 and 12.

[0080] Blocks 20 and 21 of rows 9 and 10 are separated by grooves 24 and25 which extend in an oblique (diagonal) direction relative toequatorial plane 100. Each oblique groove 24 has a rectilinear axisinclined at about 20° relative to the equatorial plane. Each obliquegroove 25 also has a rectilinear axis. The axes of oblique grooves 24and 25 are substantially perpendicular to each other and form an angleof about 105°, which varies by about ±10°. Each oblique groove 24crosses two successive oblique grooves 25 and forms with the first aT-shaped junction 26 and with the second a T-shaped junction 27, i.e.two dead-end junctions.

[0081] Oblique grooves 24 have a portion 124 of restricted width and aportion 224 whose width increases in the direction of circumferentialside groove 13. These have a substantially divergent shape.

[0082] Pairs of blocks 21 are also separated by narrow oblique grooves28, or by oblique sipes 29, substantially parallel to oblique grooves25. Oblique grooves 28 and oblique sipes 29 are in communication withcircumferential groove 7 and alternate in the circumferentialdevelopment of the tyre. Each oblique groove 28 joins up with that endof a respective oblique groove 24 which is nearest equatorial plane 100,by means of an elbow join. In turn, each oblique sipe 29 joins up withthat end of a respective oblique groove 24 which is nearest equatorialplane 100.

[0083] Each trapezoid-shaped block 20 has a sipe 39 which is essentiallytrapezoid-shaped. Each trapezoid-shaped sipe 39 has a similar shape tothat of respective trapezoid-shaped block 20 and is smaller in size thanthis block.

[0084] Blocks 21 are free of sipes.

[0085] Blocks 22 and 23 of rows 11 and 12 are separated by grooves 30and 31 which extend in an oblique direction relative to equatorial plane100. Each oblique groove 30 has a rectilinear axis inclined at an angleof about 20° relative to the equatorial plane. Each oblique groove 31also has a rectilinear axis. The axes of oblique grooves 30 and 31 areessentially perpendicular and form an angle of about 105°±10°. Eachoblique groove 30 crosses two successive oblique grooves 31 and forms aT-shaped junction 32 with the first and a T-shaped junction 33 with thesecond, i.e. two dead-end junctions.

[0086] Oblique grooves 30 have a narrow portion 130 and a portion 230whose width increases in the direction of circumferential side groove14. These portions have a substantially divergent shape.

[0087] Pairs of blocks 22 are also separated by narrow oblique grooves34, or by oblique sipes 35, which are substantially parallel to obliquegrooves 31. Grooves 34 and sipes 35 are in communication withcircumferential groove 8 and alternate in the circumferentialdevelopment of the tyre. Each oblique groove 34 joins up with that endof a respective oblique groove 30 which is nearest equatorial plane 100,by means of an elbow join. In turn, each oblique sipe 35 joins up withthat end of a respective oblique groove 30 which is nearest equatorialplane 100,

[0088] Each block 22 has two oblique sipes 36, which are substantiallyparallel to oblique grooves 31, and a sipe 37 which is essentiallyhook-shaped with sides which are substantially parallel to obliquegrooves 30 and 31.

[0089] Each trapezoid-shaped block 23 has three oblique sipes 38, whichare substantially parallel to oblique grooves 31.

[0090] Blocks 20 are equal in area to blocks 23 and blocks 21 are equalin area to blocks 22.

[0091] Preferably, the rows of blocks 22 and 23 are located in the innerzone of the tread (vehicle side), when the tyre is mounted on the motorvehicle.

[0092] Each block 22 of row 11 is obtained by rotating a block 21 of row10 by 180° around an axis lying in the plane of the sheet and passingthrough equatorial plane 100. The block thus obtained is then turnedover by 180° relative to an axis lying in the plane of the sheet andperpendicular to equatorial plane 100. In turn, each block 23 of row 12is obtained in the same way from a block 20 of row 9.

[0093] The row of shoulder blocks 15 comprises shoulder blocks 40, eachof which is formed from a portion which is substantiallyrectangular-shaped and from a portion which is substantiallyparallelogram-shaped. Blocks 40 are separated from each other by grooves41 communicating with circumferential side groove 13. Each transversegroove 41 is formed from a portion which is essentially transverserelative to equatorial plane 100 and from an oblique connecting portion.The two portions are continuous and each oblique portion has an opening140 which is essentially concentric with one of oblique grooves 25 ofthe rows of blocks 9 and 10. Each block 40 has a sipe 42 formed from anessentially transverse portion and from an oblique portion.

[0094] The row of shoulder blocks 16 comprises shoulder blocks 45, eachof which is formed from a portion which is essentiallyrectangular-shaped and from a portion which is essentiallyparallelogram-shaped. Blocks 45 are separated from each other by grooves46 communicating with circumferential side groove 14. Each transversegroove 46 is formed from a portion which is essentially transverserelative to equatorial plane 100 and from an oblique connecting portion.The two portions are continuous and each oblique portion has an opening146 which is essentially eccentric relative to oblique grooves 31 of therows of blocks 11 and 12. Each block 45 has an essentially transversesipe 47, and two sipes 48 located on either side of sipe 47. Sipes 48are formed from an oblique portion and from an essentially transverseportion.

[0095] In tyre 1, groove 7 is in communication with groove 13 viatortuous zig-zag transverse paths comprising oblique grooves 28, 24 and25. In turn, groove 8 is in communication with groove 14 via tortuouszig-zag transverse paths comprising oblique grooves 34, 30 and 31.

[0096] Circumferential groove 14 is wider than circumferential groove 13and circumferential grooves 7 and 8 are wider than groove 14. Forexample, groove 13 has a width of between 3 mm and 5.5 mm; groove 14 hasa width of between 4 mm and 7 mm and grooves 7 and 8 have a width ofbetween 9 mm and 11 mm.

[0097] Oblique grooves 31 are wider than grooves 24. For example,oblique grooves 24 have widths of between 4 mm and 6 mm and obliquegrooves 31 have widths of between 4.5 mm and 6.5 mm.

[0098] Transverse grooves 46 are wider than transverse grooves 41. Forexample, grooves 41 have a width of between 5 mm and 6.5 mm and grooves46 have a width of between 5.5 mm and 7 mm.

[0099] Grooves 7, 8, 13 and 14 have a depth of between 7 mm and 9.5 mm.

[0100]FIGS. 4, 5 and 6 show a multipurpose tyre 51 of asymmetric typewhich constitutes a variant of tyre 1, and in which parts identical tothose of tyre 1 are indicated by the same numerals. Tyre 51 has a tread52 in which circumferential rib 6 has oblique sipes 53 and 54. Eachblock 21 has three oblique sipes 55. Each trapezoid-shaped block 20 hastwo oblique sipes 56 and a chamfered edge. Each trapezoid-shaped block23 also has a chamfered edge.

[0101] Preferably, the rows of blocks 22 and 23 are located in the innerzone of the tread (vehicle side), when the tyre is mounted on the motorvehicle.

[0102] Each shoulder block 40 has a sipe 57 formed from a transverseportion and from an oblique portion, and two transverse sipes 58 locatedon either side of sipe 57. Each shoulder block 40 and 45 has a chamferededge.

[0103]FIGS. 7 and 8 show a multipurpose tyre 61 which constitutes afurther variant of tyre 1, and parts which are identical are indicatedby the same numerals. Tyre 61 has a tread 62 of asymmetric type. Intread 62, circumferential rib 6 has a mid-plane 63 which is eccentricrelative to equatorial plane 100.

[0104] The rows of blocks 9 and 12 comprise blocks 64 and 67,respectively, which are essentially trapezoid-shaped (scalene trapezium)with three curved sides and three rounded edges. The rows of blocks 10and 11 comprise blocks 65 and 66, respectively, with two portions whichextend obliquely relative to each other, such that each block has anoverall shape resembling that of the number 1, with three curved sidesand four rounded edges. The rows of shoulder blocks 15 and 16 compriseshoulder blocks 68 and 69, respectively, which are essentiallyparallelogram-shaped with two curved sides and two rounded edges.

[0105] Blocks 64 and 65 of rows 9 and 10 are separated by curved obliquegrooves 70 and 71. Each groove 70 has an axis inclined at an angle ofabout 20° relative to the equatorial plane. The axes of grooves 70 and71 are essentially orthogonal and form an angle of about 105°±10°. Eachoblique groove 70 crosses two successive oblique grooves 71 and forms aT-shaped junction 92 with the first and a T-shaped junction 93 with thesecond. Oblique grooves 70 have a width which increases in the directionof circumferential side groove 13 and are of essentially divergentshape. Oblique grooves 72 are in communication with circumferentialgroove 7. Blocks 65 are also separated by narrow oblique grooves 72which are substantially parallel to grooves 71 and are in communicationwith circumferential groove 7. Each oblique groove 72 joins up with thatend of oblique groove 70 which is nearest equatorial plane 100, by meansof an elbow join. Each trapezoid-shaped block 64 has an S-shaped sipe 80and two oblique sipes 81 located on either side of sipe 80. Each block65 has an S-shaped sipe. 82 and two oblique sipes 83.

[0106] Blocks 66 and 67 of rows 11 and 12 are separated by curvedoblique grooves 73 and 74. Each groove 73 has an axis inclined at anangle of about 20° relative to the equatorial plane. The axes of grooves73 and 74 are essentially orthogonal and form an angle of about105°±10°. Each oblique groove 73 crosses two successive oblique grooves74 and forms a T-shaped junction 94 with the first and a T-shapedjunction 95 with the second. Oblique grooves 73 have a width whichincreases in the direction of circumferential side groove 14 and are ofessentially divergent shape. Blocks 66 are also separated by narrowoblique grooves 75 which are substantially parallel to grooves 74 andare in communication with circumferential groove 8. Each oblique groove75 joins up with that end of oblique groove 73 which is nearestequatorial plane 100, by means of an elbow join. Each block 66 has anS-shaped sipe 84 and an oblique sipe 85. Each trapezoid-shaped block 67has an S-shaped sipe 86 and two oblique sipes 87 located on either sideof sipe 86.

[0107] Blocks 64 have a greater area than blocks 67 and blocks 65 have agreater area than blocks 66.

[0108] Preferably, the rows of blocks 66 and 67 are located in the innerzone of the tread (vehicle side), when the tyre is mounted on the motorvehicle.

[0109] Shoulder blocks 68 and 69 of rows 15 and 16 are separated bycurved transverse grooves 76 and 77, respectively. Each block 68 has anS-shaped sipe 88 and two essentially transverse sipes 89 located oneither side of sipe 88. Each block 69 has essentially transverse sipes90 which cross respective essentially longitudinal sipes 91 incommunication with the transverse grooves 77.

[0110]FIG. 9 shows a tread 162 of a tyre 161 which constitutes a variantof tyre 61 of FIGS. 7 and 8. Tread 162 has the same pattern as tread 62,whereas its blocks are free of sipes.

[0111]FIGS. 10 and 11 show a tread 262 of a tyre 261 and a tread 362 ofa tyre 361 which constitute other variants of tyre 61 of FIGS. 7 and 8.Treads 262 and 362 have the same pattern as tread 62, whereas the sipesin their blocks have a different shape.

[0112] The structure of tyres 1, 51, 61, 161, 261 and 361 is per se oftraditional type and comprises a carcass, a tread band encircling saidcarcass, and a pair of axially opposite side walls ending in beadsreinforced with bead wires and associated bead fillings, for fixing saidtyre to a corresponding mounting rim. The tyre preferably also comprisesa belt structure interposed between the carcass and the tread band.

[0113] The carcass is reinforced with one or more carcass plies fixed tosaid bead wires, while the belt structure comprises two belt stripsformed from sections of rubberized fabric comprising metal cords whichare parallel to each other in each strip and crossed at an angle tothose of adjacent strips, preferably inclined symmetrically relative tothe equatorial plane and radially superimposed on each other.Preferably, a third belt strip is also present, in a radially moreexternal position, provided with cords, preferably textile cords andeven more preferably cords made of a heat-shrinkable material, orientedcircumferentially, i.e. at 0° relative to said equatorial plane.

[0114] Preferably, the tyre is of the type with a very low crosssection, for example between 0.65 and 0.30, where these values indicatethe percentage value of the ratio between the height of the rightsection of the tyre and the maximum chord of said section. This ratio isusually known in the art as H/C.

[0115] Examples of tyres 1, 51, 61, 161, 261 and 361 were prepared andit was demonstrated that they had excellent values in terms oftractional power and braking on snow-covered roads and in terms ofbehaviour with respect to aquaplaning, and good handling in wet and dryconditions, by means of comparative tests against conventional tyres,carried out on roads and on tracks.

[0116] The tyres of the invention were compared with commerciallyavailable tyres of winter, summer and multipurpose types. The tread ofthe comparative tyres had the patterns X, Y, Z, V and W representedrespectively in FIGS. 12, 13, 14, 15 and 16. The first tyre (tread X) isof the summer type, the second (tread Y) is of the winter type, thethird (tread Z) is of the multipurpose (all-season) type, the fourth(tread V) is of the multipurpose type and the fifth (tread W) is of themultipurpose type.

[0117] The tyres were mounted on standard rims and inflated to therecommended working pressure.

[0118] The results of the various tests are given below.

[0119] Test I

[0120] Tractional Power on Snow

[0121] The test was carried out on the tyre of the invention shown inFIGS. 1-3, labelled C, compared with tyres with treads X, Y and Z.

[0122] In particular, the size of the tyre of the invention was 225/55R17, which is not conventional for a multipurpose tyre and was speciallydesigned, on request, for a motor vehicle manufacturer.

[0123] The size of the tyres with treads X, Y and Z was 225/60 R16, i.e.the closest size to that of the tyre of the invention, in order to beable to carry out meaningful comparative tests. In particular, theirdiameter was equal to that of the tyre of the invention.

[0124] The motor vehicle was an Audi “Hunter” 4×4 road prototype.

[0125] The test was carried out on a track covered with a layer ofcompact snow, prepared on the surface of a frozen lake.

[0126] The tests of tractional power on snow consisted of standingstarts (accelerations) carried out in first gear with clutch engaged. Bykeeping the engine speed constant (4000 or 5000 revs/min), theacceleration/force tractional power curve (the mass of the motor vehiclebeing known) was obtained as a function of slip. In the curve, the areasubtended by two predefined intervals of slip and the maximum value ofthe curve were considered.

[0127] Other tests consisted of starting from stationary in order toobtain the time and the average acceleration required to reach apredetermined velocity.

[0128] The maximum tractional power force (N) was measured and wasexpressed as a maximum percentage force (F_(max)%) relative to themaximum tractional power force of a reference tyre taken as equal to100. The integral of the force between 5 and 50% of slipping (F₅₋₅₀%)was also calculated.

[0129] The results of the abovementioned tests are given in Table Ibelow. TABLE I C X Z Y F_(max)% 124.7 100 126.3 168.4 F₅₋₅₀% 118.1 100122.7 166.7

[0130] These results show that the tractional power of the tyre of theinvention (C) is substantially equivalent to that of the bestcommercially available multipurpose tyre (Z), is much better than thatof the summer type reference tyre (X), and is only inferior to that ofthe winter tyre (Y).

[0131] The modest difference in the values of the tractional power forcefor the tyre of the invention (C) relative to those of the conventionalmultipurpose tyre (Z) is explained by the difference of the size (thetyre of the invention is of lower profile and thus more rigid) and byfewer sipes on the outer side. The result of the test on snow istherefore penalized relative to the comparative tyre.

[0132] Test II

[0133] Braking on Snow

[0134] The experimental conditions were the same as those in Test I,except that the deceleration (in m/sec²) of the car from a velocity ofbetween 50 and 10 km/h was measured and was expressed as a percentagedeceleration (a %) relative to the deceleration of a reference tyretaken as equal to 100.

[0135] The tests of braking on snow were carried out using the antilockbraking system (antiskid or ABS system), starting from an initialvelocity, for example, of 50 or 40 km/h, and ignoring the final 10 or 5km/h.

[0136] The braking distances and the average decelerations werecalculated.

[0137] The results are given in Table II below. TABLE II C X Z a% 124.5100 126.4

[0138] These results show that the braking of the tyre of the invention(C) is substantially equivalent to that of the best commerciallyavailable multipurpose tyre (Z) and is considerably superior to that ofthe reference tyre, of summer type (X).

[0139] Test III

[0140] Behaviour on Dry Terrain and Tests under Aquaplaning Conditions

[0141] With the Audi 4×4 road car fitted with the tyres described inTest I above, the behaviour on dry terrain and under aquaplaningconditions in a straight line and on a curve were evaluated.

[0142] The straight-line aquaplaning test was carried out on a straightsection of smooth asphalt of predetermined length with a layer of waterof constant predetermined depth which was replaced automatically aftereach passage of the test car.

[0143] The test of aquaplaning on a curve was carried out on a sectionof track with smooth, dry asphalt on a curve of constant radius with apredetermined length and comprising, in a final section, a zone ofpredetermined length flooded with a layer of water of predetermineddepth.

[0144] The results are shown in Table III below. TABLE III C X Z Noise(1) 7-8 7-8 Noise (2) O.K. O.K. Handling (3) 98.1% 100% 95.1% Steeringpad (3) 98.8% 100% 97.7% Aquaplaning on a 89.0% 100% 93.2% curve (4)Aquaplaning in a 92.8% 100% 97.9% straight line (5) Aquaplaning on a72.4% 100% curve (6) Aquaplaning in a 93.5% 100% 103.5%  straight line(7) Braking (8) 98.3% 100% 95.5% Handling (9) Steering-accuracy  6.5-7.5 7-7.5 7 Stability 7.5-8 7-7.5 6.5-7   Driving comfort 7.5-87.5   7-7.5

[0145] These results show that the tyre of the invention exhibitsbehaviour under dry and wet conditions, in particular a track stability,which is virtually of the same level as that of a summer tyre (X),although it has a tread of clearly multipurpose type. Specifically, itcan be seen that these results are overall better than those of the bestmultipurpose reference tyre (Z), except in the aquaplaning tests, forthe reason already explained with reference to Test I, i.e. on accountof the lower profile of the tyre of the invention which has a widerimprint area and is thus more susceptible to aquaplaning.

[0146] Test IV

[0147] Tractional Power on Snow

[0148] The test was carried out on the tyres of the invention shown inFIGS. 7 and 8 compared with tyres of multipurpose type with treads V andW. All the tyres tested were of the size 205/55 R16.

[0149] In particular, the tyres of the invention were labelled asfollows:

[0150] E: FIGS. 7 and 8;

[0151] Base (1st variant): tread pattern of FIGS. 7-8, but without sipes(FIG. 9);

[0152] E1 (2nd variant): tread pattern of FIGS. 7-8, but with differentsipes (FIG. 10);

[0153] E2 (3rd variant): tread pattern of FIGS. 7-8, but with differentsipes (FIG. 11).

[0154] All the tyres of the invention had a tread made of the samecompound.

[0155] The tests were carried out using a Mercedes Benz C280 carweighing 1490 kg and fitted with ABS.

[0156] The test conditions were the same as those described in Test Iabove.

[0157] The results of the test of tractional power on snow are given inTable IV. TABLE IV Base E E1 E2 W V F_(max)% 100.0 125.2 145.7 141.9115.1 107.9 F₅₋₅₀% 100.0 141.8 164.2 161.7 130.3 120.6

[0158] These results show that the tractional power properties of thetreads E, E1 and E2 of the invention are better than those of both thecomparative tyres.

[0159] Test V

[0160] Braking under Wet Conditions

[0161] The car was that of Test IV fitted with tyres with a tread of theinvention of type E and comparative tyres of type W.

[0162] The results are given in Table V below. TABLE V E W A% 100.0 96.9

[0163] These results show that the tread E of the invention gives betterresults than the comparative tread of W type.

[0164] Test VI

[0165] Aquaplaning in a Straight Line

[0166] The test was carried out as described above (Test III). At first,the speed (km/h) of initial loss of adhesion of the tyres (V. init.) wasrecorded, then the speed (km/h) of total loss of adhesion (V. end) wasrecorded.

[0167] The aquaplaning test was carried out using a Mercedes Benz C280car fitted with tyres of the E type of the invention and comparativetyres of the V and W type.

[0168] The results of the test are shown in Table VI TABLE VI E W V V.init. 76.6 74.6 71.8 V. end 82.5 79.2 77.0

[0169] These results show that the straight-line aquaplaning propertiesof the tyre with the tread E of the invention are better than those ofthe comparative tyres.

1. Multipurpose tyre for a motor vehicle, said tyre having an equatorialplane (100) and being provided with a tread (2; 52; 62; 162; 262; 362)having a central region (3) and a first and a second shoulder region (5,4), said central region (3) being separated from said first and secondshoulder regions (5, 4) by a first and a second circumferential groove(14, 13), respectively, said central region (3) comprising at leastfirst and second blocks (22, 23; 66, 67) arranged between saidequatorial plane (100) and one (14) of said first and secondcircumferential grooves, characterized in that said first and secondblocks (22, 23; 66, 67) form a first and a second row (11, 12) and areseparated from each other by first and second grooves (30, 31; 73, 74)which extend in an oblique direction relative to said equatorial plane(100), said first oblique grooves (30; 73) extending substantiallyperpendicularly relative to said second oblique grooves (31; 74), andeach first oblique groove (30; 73) crossing two successive secondoblique grooves (31; 74) and forming with each of them a respectivejunction which is essentially T-shaped (32, 33; 94, 95).
 2. Tyreaccording to claim 1, characterized in that said first oblique grooves(30; 73) have a width which decreases in the direction of saidequatorial plane (100).
 3. Tyre according to claim 1, characterized inthat said first and second blocks (22, 23; 66, 67) of said first andsecond rows (11, 12) are side by side and diverge obliquely.
 4. Tyreaccording to claim 1, characterized in that said first blocks (22; 66)have two portions which extend obliquely relative to each other, suchthat each block has an overall shape resembling that of the number
 1. 5.Tyre according to claim 1, characterized in that said second blocks (23;67) are essentially trapezoid-shaped.
 6. Tyre according to claim 1,characterized in that said central region (3) also comprises third andfourth blocks (21, 20; 65, 64) located between said equatorial plane(100) and the other (13) of said first and second circumferentialgrooves, said third and fourth blocks forming a third and a fourth row(10, 9) and being separated from each other by third and fourth grooves(24, 25; 70, 71) which extend in an oblique direction relative to saidequatorial plane (100), said third oblique grooves (24; 70) extendingsubstantially perpendicularly relative to said fourth oblique grooves(25; 71), each third oblique groove (24; 70) crossing two successivefourth oblique grooves (25; 71) and forming with each of them arespective junction which is essentially T-shaped (26, 27; 92, 93). 7.Tyre according to claim 6, characterized in that said third obliquegrooves (24; 70) have a width which decreases in the direction of saidequatorial plane (100).
 8. Tyre according to claim 6, characterized inthat said third and fourth blocks (21, 20; 65,64) of said third andfourth rows (10, 9) are side by side and diverge obliquely, said thirdand fourth side-by-side blocks (21, 20; 65, 64) being inclined in theopposite direction to said first and second side-by-side blocks (22, 23;66, 67).
 9. Tyre according to claim 6, characterized in that said thirdblocks (21; 65) have two portions which extend obliquely relative toeach other, such that each block has an overall shape resembling that ofthe number
 1. 10. Tyre according to claim 6, characterized in that saidfourth blocks (20; 64) are essentially trapezoid-shaped.
 11. Tyreaccording to claim 1, characterized in that at least two of said firstblocks (22; 66) are also separated from each other by a fifth obliquegroove (34; 75) which is substantially parallel to said second obliquegrooves (31; 74), said fifth oblique groove (34; 75) joining up with oneof said first oblique grooves (30; 73) at that end of the latter whichis nearest said equatorial plane (100), by means of an elbow join. 12.Tyre according to claim 1, characterized in that at least two of saidfirst blocks (22) are also separated from each other by a first obliquesipe (35) which is substantially parallel to said second oblique grooves(31), said first oblique sipe (35) joining up with one of said firstoblique grooves (30) at that end of the latter which is nearest saidequatorial plane (100).
 13. Tyre according to claim 6, characterized inthat at least two of said third blocks (21; 65) are also separated fromeach other by a sixth oblique groove (28; 72) which is substantiallyparallel to said fourth oblique grooves (25; 71), said sixth obliquegroove (28; 72) joining up with one of said third oblique grooves (24;70) at that end of the latter which is nearest said equatorial plane(100), by means of an elbow join.
 14. Tyre according to claim 6,characterized in that at least two of said third blocks (21) are alsoseparated from each other by a second oblique sipe (29) which issubstantially parallel to said fourth oblique grooves (25), said secondoblique sipe (29) joining up with one of said third oblique grooves (24)at that end of the latter which is nearest said equatorial plane (100).15. Tyre according to claim 1, characterized in that said central region(3) also comprises a circumferential rib (6) and a third and a fourthcircumferential groove (8, 7).
 16. Tyre according to claims 11 and 15,characterized in that said third circumferential groove (8) separatessaid circumferential rib (6) from said row of first blocks (22; 66) andis in communication with said first circumferential groove (14) viafirst transverse zig-zag paths comprising said fifth oblique grooves(34; 75) and said first and second oblique grooves (30, 31; 73, 74). 17.Tyre according to claims 13 and 15, characterized in that said fourthcircumferential groove (7) separates said circumferential rib (6) fromsaid row of third blocks (21; 65) and is in communication with saidsecond circumferential groove (13) via second transverse zig-zag pathscomprising said sixth oblique grooves (28; 72) and said third and fourthoblique grooves (24, 25; 70, 71).
 18. Tyre according to claim 1,characterized in that said first circumferential groove (14) is widerthan the width of said second circumferential groove (13).
 19. Tyreaccording to claim 6, characterized in that said second oblique grooves(31) are wider than the width of said fourth oblique grooves (25). 20.Tyre according to claim 15, characterized in that said third and fourthcircumferential grooves (8, 7) are wider than the width of said firstcircumferential groove (14).
 21. Tyre according to claim 6,characterized in that said first and second blocks (22, 23) have areasthat are substantially equal to the areas of said third and said fourthblocks (21, 20), respectively.
 22. Tyre according to claim 6,characterized in that said first and second blocks (66, 67) have areasthat are substantially less than the areas of said third and said fourthblocks (65, 64), respectively.
 23. Tyre according to claim 15,characterized in that said circumferential rib (6) has a mid-plane whichis coincident with said equatorial plane (100).
 24. Tyre according toclaim 15, characterized in that said circumferential rib (6) has amid-plane (63) which is eccentric with respect to said equatorial plane(100).
 25. Tyre according to claim 1, characterized in that each of saidfirst blocks (22) has a pair of substantially oblique third sipes (36)which are substantially parallel to said second oblique grooves (31),and a fourth sipe (37) which is essentially hook-shaped with sides thatare substantially parallel to said first and said second oblique grooves(30, 31).
 26. Tyre according to claim 1, characterized in that each ofsaid second blocks (23) has three substantially oblique fifth sipes (38)which are substantially parallel to said second oblique grooves (31).27. Tyre according to claim 6, characterized in that each of said thirdblocks (21) is free of sipes.
 28. Tyre according to claim 6,characterized in that each of said fourth blocks (20) has a sixth sipe(39) which is substantially trapezoid-shaped, similar to the shape ofthe respective fourth block which is substantially trapezoid-shaped (20)and smaller in size than this block.
 29. Tyre according to claim 6,characterized in that each of said third blocks (21) has threesubstantially oblique seventh sipes (55) which are substantiallyparallel to said fourth oblique grooves (25).
 30. Tyre according toclaim 6, characterized in that each of said fourth blocks (20) has apair of substantially oblique eighth sipes (56) which are substantiallyparallel to said fourth oblique grooves (25).
 31. Tyre according toclaim 1, characterized in that each of said first blocks (66) has aninth oblique sipe (85) which is substantially parallel to said secondoblique grooves (74), and a tenth sipe which is substantially S-shaped.32. Tyre according to claim 1, characterized in that each of said secondblocks (67) has an eleventh central sipe (86) which is substantiallyS-shaped, and two substantially oblique twelfth sipes (87) which aremore less parallel to said second oblique grooves (74) and located oneither side of said S-shaped eleventh sipe (86).
 33. Tyre according toclaim 6, characterized in that each of said third blocks (65) has twosubstantially oblique thirteenth sipes (83) which are substantiallyparallel to said fourth oblique grooves (71), and a fourteenth sipe (82)which is substantially S-shaped.
 34. Tyre according to claim 6,characterized in that each of said fourth blocks (64) has a centralfifteenth sipe (80) which is substantially S-shaped, and twosubstantially oblique sixteenth sipes (81) which are substantiallyparallel to said fourth oblique grooves (71) and located on either sideof said S-shaped fifteenth sipe (80).
 35. Tyre according to claim 15,characterized in that said circumferential rib (6) has seventeenth andeighteenth substantially oblique sipes (53, 54) which are substantiallyparallel to said second and fourth oblique grooves (31, 25).
 36. Tyreaccording to claim 1, characterized in that said first shoulder region(5) comprises fifth shoulder blocks (45; 69) separated by seventhgrooves (46; 77) which are substantially transverse relative to saidequatorial plane (100) and communicate with said first circumferentialgroove (14), and said second shoulder region (4) comprises sixthshoulder blocks (40; 68) separated by eighth grooves (41; 76) that aresubstantially transverse relative to said equatorial plane (100) andcommunicate with said second circumferential groove (13).
 37. Tyreaccording to claim 36, characterized in that each seventh substantiallytransverse groove (46; 77) has a connecting portion with an openingwhich is substantially eccentric relative to each second oblique groove(31; 74).
 38. Tyre according to claims 6 and 36, characterized in thateach substantially transverse eighth groove (41; 76) has a connectingportion with an opening which is substantially concentric with a fourthoblique groove (25; 71).
 39. Tyre according to claim 36, characterizedin that each substantially transverse seventh groove (46; 77) is widerthan the width of each substantially transverse eighth groove (41; 76).40. Tyre according to claim 36, characterized in that each of said fifthshoulder blocks (45) has three nineteenth sipes (47, 48) which aresubstantially transverse relative to said equatorial plane (100). 41.Tyre according to claim 36, characterized in that each of said sixthshoulder blocks (40) has a twentieth sipe (42) which is substantiallytransverse relative to said equatorial plane (100).
 42. Tyre accordingto claim 36, characterized in that each of said sixth shoulder blocks(40) has three twenty-first sipes (57, 58) which are substantiallytransverse relative to said equatorial plane (100).
 43. Tyre accordingto claim 36, characterized in that each of said fifth shoulder blocks(69) has two twenty-second substantially transverse sipes (90), each ofwhich crosses a substantially circumferential twenty-third sipe (91).44. Tyre according to claim 36, characterized in that each of said sixthshoulder blocks (68) has a central twenty-fourth sipe (88) which issubstantially S-shaped and two substantially transverse twenty-fifthsipes (89) located on either side of said S-shaped twenty-fourth sipe(88).
 45. Multipurpose tyre for a motor vehicle, said tyre having anequatorial plane (100) and being provided with a tread (2; 52; 62; 162;262; 362) with a central region (3) and a first and a second shoulderregion (5, 4), said central region (3) being separated from said firstand second shoulder regions (5, 4) by a first and a secondcircumferential groove (14, 13), respectively, said central region (3)comprising at least first and second blocks (22, 23; 66, 67) locatedbetween said equatorial plane (100) and one (14) of said first andsecond circumferential grooves, characterized in that said first andsecond blocks (22, 23; 66, 67) form a first and a second row (11, 12)and are separated from each other by first and second grooves (30, 31;73, 74) which extend in an oblique direction relative to said equatorialplane (100), said first oblique grooves (30; 73) extending essentiallyperpendicularly relative to said second oblique grooves (31; 74), saidfirst oblique grooves (30; 73) having a width which decreases in thedirection of said equatorial plane (100).
 46. Tyre according to claim45, characterized in that said central region (3) also comprises thirdand fourth blocks (21, 20; 65, 64) located between said equatorial plane(100) and the other (13) of said first and second circumferentialgrooves, said third and fourth blocks (21, 20; 65, 64) forming a thirdand a fourth row (10, 9) and being separated from each other by thirdand fourth grooves (24, 25; 70, 71) which extend in an oblique directionrelative to said equatorial plane (100), said third oblique grooves (24;70) extending essentially perpendicularly relative to said fourthoblique grooves (25; 71), said third oblique grooves (24; 70) having awidth which decreases in the direction of said equatorial plane (100).